Turbochargers have become popular for many different types of internal combustion engines, from large diesel engines to small gasoline engines. The purpose of the turbocharger in all of them is to provide a high pressure charge of a fluid or gas, typically air, to the combustion chamber of the engine. The turbocharger is typically driven by the exhaust of the engine, which is used to drive a rotatively-driven compressor that compresses the air or fluid that is introduced to the combustion chamber of the engine. As the pressure in the combustion chamber goes up, so does the pressure of the exhaust, creating a feedback loop that can create an overload condition for either the turbocharger or the engine.
To control the turbocharger so that it does not create an overload condition, a waste gate valve is typically employed in the exhaust circuit that diverts all or part of the exhaust gas away from the turbine drive of the compressor, so as to limit the pressure that the turbine of the turbocharger is subjected to. Thereby, the boost pressure that the turbocharger provides to the engine is limited at a maximum level to avoid damage to the engine or turbocharger.
In some turbocharger systems, two or more turbochargers are employed to operate under different conditions of the engine. A smaller, lower flow turbocharger will operate for lower engine speeds or lower load conditions of the engine, and a larger higher flow turbocharger will operate for higher engine speeds or more demanding conditions of the engine. These are known as turbocharger sequencing applications and may require several valves in the exhaust lines between the two turbochargers to direct exhaust to one or the other of the turbochargers, or to bypass one or both of them.
The valves that are used in turbocharger applications are subjected to extremely severe operating conditions, as they must operate over a large temperature range (typically −40° C.-800° C., sometimes up to 1000° C.), since the exhaust is extremely hot, and the exhaust contains corrosive and acidic materials. These valves, particularly valves in turbocharger sequencing applications, must have very low leakage characteristics so that exhaust gas does not escape to the engine compartment or elsewhere and, particularly for turbocharger sequencing applications, to improve the efficiency of the system. As a result of this requirement, most prior art turbocharger system exhaust valves have been poppet type valves, which traditionally leak less than butterfly valves.
Another consideration of these types of valves, in addition to maintaining low leakage through a wide temperature range, is maintaining low hysteresis through a wide temperature range. The valve is typically actuated by a pressure operated actuator and so the force that the valve exerts on the actuator at a given boost pressure should be the same whether the valve is being opened or being closed. That is, the relationship of the force required for a given opening of the valve should be the same, or as nearly the same as possible, whether the valve is being opened or being closed.
In addition, typically such valves are actuated in only one direction, either open or closed, and in the other direction are actuated by a spring. It is desirable to make the force of the spring as low as possible, while still ensuring complete actuation of the valve, for example, if the spring biases the valve closed, as is typical, then when biased closed the valve should be completely closed, and not excessively leak.